At the age of 13 is it normal to have menstruation 2 times a month or you should be worried.

It's often normal during the teen years to have irregular periods. When a girl's period is irregular, it might come more than once a month or only come every few months. Some girls find they get their period more than once some months, then they might not have another for several months.

Explanation:

there is no

explanation:

multiple allele inheritance is traits controlled by more than 2 alleles (ex: pigeons having 3 alleles of one gene govern their feather color).   so the answer is a.